In a conventional magnetic data memorizing system such as a floppy disc drive or a hard disc drive, residual magnetism as data on a surface of the disc is detected to be converted to an electric signal while the disc is rotated by a motor such as a spindle motor. The motor is driven by a driving circuit including transistors to rotate at a constant rotation speed. Such a transistor generates noises when it becomes at ON or OFF state. However, a voltage induced in a magnetic head thereof is as weak as usually up to 1 mV at the most, so that it is required to avoid generation of noises. Therefore, the motor is controlled by a so-called linear mode control in which the transistor of the driving circuit operates within an unsaturated (linear) operation region.
According to the conventional magnetic data memorizing system, however, there is a disadvantage in that the power loss is large especially at a starting state at which a large driving current is required, so that it is difficult to increase an average current of an integrated circuit (IC) including the driving circuit for a motor in the magnetic data memorizing system. Further, it is difficult to rise the rotation speed of the motor quickly at the starting state.